1 .TH CJPEG 1 "18 January 2013"
3 cjpeg \- compress an image file to a JPEG file
16 compresses the named image file, or the standard input if no file is
17 named, and produces a JPEG/JFIF file on the standard output.
18 The currently supported input file formats are: PPM (PBMPLUS color
19 format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster
20 Toolkit format). (RLE is supported only if the URT library is available.)
22 All switch names may be abbreviated; for example,
28 Most of the "basic" switches can be abbreviated to as little as one letter.
29 Upper and lower case are equivalent (thus
33 British spellings are also accepted (e.g.,
35 though for brevity these are not mentioned below.
37 The basic switches are:
39 .BI \-quality " N[,...]"
40 Scale quantization tables to adjust image quality. Quality is 0 (worst) to
41 100 (best); default is 75. (See below for more info.)
44 Create monochrome JPEG file from color input. Be sure to use this switch when
45 compressing a grayscale BMP file, because
47 isn't bright enough to notice whether a BMP file uses only shades of gray.
50 you'll get a smaller JPEG file that takes less time to process.
54 Using this switch suppresses the conversion from RGB
55 colorspace input to the default YCbCr JPEG colorspace.
58 Perform optimization of entropy encoding parameters. Without this, default
59 encoding parameters are used.
61 usually makes the JPEG file a little smaller, but
63 runs somewhat slower and needs much more memory. Image quality and speed of
64 decompression are unaffected by
68 Create progressive JPEG file (see below).
71 Input file is Targa format. Targa files that contain an "identification"
72 field will not be automatically recognized by
74 for such files you must specify
78 treat the input as Targa format.
79 For most Targa files, you won't need this switch.
83 switch lets you trade off compressed file size against quality of the
84 reconstructed image: the higher the quality setting, the larger the JPEG file,
85 and the closer the output image will be to the original input. Normally you
86 want to use the lowest quality setting (smallest file) that decompresses into
87 something visually indistinguishable from the original image. For this
88 purpose the quality setting should be between 50 and 95; the default of 75 is
89 often about right. If you see defects at
91 75, then go up 5 or 10 counts at a time until you are happy with the output
92 image. (The optimal setting will vary from one image to another.)
95 100 will generate a quantization table of all 1's, minimizing loss in the
96 quantization step (but there is still information loss in subsampling, as well
97 as roundoff error). This setting is mainly of interest for experimental
98 purposes. Quality values above about 95 are
100 recommended for normal use; the compressed file size goes up dramatically for
101 hardly any gain in output image quality.
103 In the other direction, quality values below 50 will produce very small files
104 of low image quality. Settings around 5 to 10 might be useful in preparing an
105 index of a large image library, for example. Try
107 2 (or so) for some amusing Cubist effects. (Note: quality
108 values below about 25 generate 2-byte quantization tables, which are
109 considered optional in the JPEG standard.
111 emits a warning message when you give such a quality value, because some
112 other JPEG programs may be unable to decode the resulting file. Use
114 if you need to ensure compatibility at low quality values.)
116 The \fB-quality\fR option has been extended in this version of \fBcjpeg\fR to
117 support separate quality settings for luminance and chrominance (or, in
118 general, separate settings for every quantization table slot.) The principle
119 is the same as chrominance subsampling: since the human eye is more sensitive
120 to spatial changes in brightness than spatial changes in color, the chrominance
121 components can be quantized more than the luminance components without
122 incurring any visible image quality loss. However, unlike subsampling, this
123 feature reduces data in the frequency domain instead of the spatial domain,
124 which allows for more fine-grained control. This option is useful in
125 quality-sensitive applications, for which the artifacts generated by
126 subsampling may be unacceptable.
128 The \fB-quality\fR option accepts a comma-separated list of parameters, which
129 respectively refer to the quality levels that should be assigned to the
130 quantization table slots. If there are more q-table slots than parameters,
131 then the last parameter is replicated. Thus, if only one quality parameter is
132 given, this is used for both luminance and chrominance (slots 0 and 1,
133 respectively), preserving the legacy behavior of cjpeg v6b and prior.
134 More (or customized) quantization tables can be set with the \fB-qtables\fR
135 option and assigned to components with the \fB-qslots\fR option (see the
136 "wizard" switches below.)
138 JPEG files generated with separate luminance and chrominance quality are fully
139 compliant with standard JPEG decoders.
142 For this setting to be useful, be sure to pass an argument of \fB-sample 1x1\fR
143 to \fBcjpeg\fR to disable chrominance subsampling. Otherwise, the default
144 subsampling level (2x2, AKA "4:2:0") will be used.
148 switch creates a "progressive JPEG" file. In this type of JPEG file, the data
149 is stored in multiple scans of increasing quality. If the file is being
150 transmitted over a slow communications link, the decoder can use the first
151 scan to display a low-quality image very quickly, and can then improve the
152 display with each subsequent scan. The final image is exactly equivalent to a
153 standard JPEG file of the same quality setting, and the total file size is
154 about the same --- often a little smaller.
156 Switches for advanced users:
159 Use arithmetic coding.
161 arithmetic coded JPEG is not yet widely implemented, so many decoders will be
162 unable to view an arithmetic coded JPEG file at all.
165 Use integer DCT method (default).
168 Use fast integer DCT (less accurate).
171 Use floating-point DCT method.
172 The float method is very slightly more accurate than the int method, but is
173 much slower unless your machine has very fast floating-point hardware. Also
174 note that results of the floating-point method may vary slightly across
175 machines, while the integer methods should give the same results everywhere.
176 The fast integer method is much less accurate than the other two.
179 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
180 attached to the number.
182 (the default) means no restart markers.
185 Smooth the input image to eliminate dithering noise. N, ranging from 1 to
186 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
189 Set limit for amount of memory to use in processing large images. Value is
190 in thousands of bytes, or millions of bytes if "M" is attached to the
193 selects 4000000 bytes. If more space is needed, temporary files will be used.
195 .BI \-outfile " name"
196 Send output image to the named file, not to standard output.
199 Compress to memory instead of a file. This feature was implemented mainly as a
200 way of testing the in-memory destination manager (jpeg_mem_dest()), but it is
201 also useful for benchmarking, since it reduces the I/O overhead.
204 Enable debug printout. More
206 give more output. Also, version information is printed at startup.
214 option inserts extra markers that allow a JPEG decoder to resynchronize after
215 a transmission error. Without restart markers, any damage to a compressed
216 file will usually ruin the image from the point of the error to the end of the
217 image; with restart markers, the damage is usually confined to the portion of
218 the image up to the next restart marker. Of course, the restart markers
219 occupy extra space. We recommend
221 for images that will be transmitted across unreliable networks such as Usenet.
225 option filters the input to eliminate fine-scale noise. This is often useful
226 when converting dithered images to JPEG: a moderate smoothing factor of 10 to
227 50 gets rid of dithering patterns in the input file, resulting in a smaller
228 JPEG file and a better-looking image. Too large a smoothing factor will
229 visibly blur the image, however.
231 Switches for wizards:
234 Force baseline-compatible quantization tables to be generated. This clamps
235 quantization values to 8 bits even at low quality settings. (This switch is
236 poorly named, since it does not ensure that the output is actually baseline
237 JPEG. For example, you can use
243 .BI \-qtables " file"
244 Use the quantization tables given in the specified text file.
246 .BI \-qslots " N[,...]"
247 Select which quantization table to use for each color component.
249 .BI \-sample " HxV[,...]"
250 Set JPEG sampling factors for each color component.
253 Use the scan script given in the specified text file.
255 The "wizard" switches are intended for experimentation with JPEG. If you
256 don't know what you are doing, \fBdon't use them\fR. These switches are
257 documented further in the file wizard.txt.
260 This example compresses the PPM file foo.ppm with a quality factor of
261 60 and saves the output as foo.jpg:
268 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
269 compressing full-color (24-bit) images. In particular, don't try to convert
270 cartoons, line drawings, and other images that have only a few distinct
271 colors. GIF works great on these, JPEG does not. If you want to convert a
272 GIF to JPEG, you should experiment with
277 options to get a satisfactory conversion.
279 or so is often helpful.
281 Avoid running an image through a series of JPEG compression/decompression
282 cycles. Image quality loss will accumulate; after ten or so cycles the image
283 may be noticeably worse than it was after one cycle. It's best to use a
284 lossless format while manipulating an image, then convert to JPEG format when
285 you are ready to file the image away.
291 is worth using when you are making a "final" version for posting or archiving.
292 It's also a win when you are using low quality settings to make very small
293 JPEG files; the percentage improvement is often a lot more than it is on
294 larger files. (At present,
296 mode is always selected when generating progressive JPEG files.)
300 If this environment variable is set, its value is the default memory limit.
301 The value is specified as described for the
305 overrides the default value specified when the program was compiled, and
306 itself is overridden by an explicit
317 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
318 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
320 Independent JPEG Group
322 This file was modified by The libjpeg-turbo Project to include only information
323 relevant to libjpeg-turbo, to wordsmith certain sections, and to describe
324 features not present in libjpeg.
326 Support for GIF input files was removed in cjpeg v6b due to concerns over
327 the Unisys LZW patent. Although this patent expired in 2006, cjpeg still
328 lacks GIF support, for these historical reasons. (Conversion of GIF files to
329 JPEG is usually a bad idea anyway.)
331 Not all variants of BMP and Targa file formats are supported.
335 switch is not a bug, it's a feature. (It would be a bug if the Targa format
336 designers had not been clueless.)
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